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IBM to put 48Mbytes on next-gen microprocessor

Posted: 19 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:IBM? DRAM? 45nm technology? SRAM cache? Intel?

IBM Corp. detailed a method for tripling the amount of memory on a microprocessor, potentially doubling its performance. By combining techniques in process and circuit design, IBM believes it can put as much as 48Mbytes of fast DRAM on a reasonably sized CPU when its 45nm technology becomes available in 2008.

IBM's upcoming Power6 CPUs use 8Mbytes SRAM cache. Rival Intel Corp.'s Itanium processors use as much as 18Mbytes.

"Processors are definitely cache starved, and as you go more towards multi-core processors, the need for memory integration becomes more acute," said Subramanian Iyer, a distinguished engineer and director of 45nm technology development at IBM. "There are some server chips that could not be made without this technology," he added.

In a paper at the International Solid State Circuits Conference last Feb. 14, IBM described a 65nm prototype embedded DRAM with a latency of just 1.5ns and a cycle time of 2ns. That's an order of magnitude faster than today's DRAMs and competitive with SRAM that is typically used for microprocessor cache memory.

"To put 24-36Mbytes of memory on a chip, you would need a 600mm? die today. Using this technology you could put that much memory on a 300-350mm? die," Iyer said.

IBM expects to use the technique on its future Power and Cell processors as well as have it available for its ASIC customers. "It's being defined in a way that it can be part of our standard 45nm process technology," Iyer said.

IBM combined two advances to enable the new memory integration. The company found a way to migrate its deep trench technology used for DRAMs from CMOS to its silicon-on-insulator (SOI) logic process. In a paper last December, IBM described that work that involved suppressing the floating-body effect in SOI.

"Our entire processor road map is based on SOI," said Iyer.

New circuit designs use short bit lines to eliminate the need for sense amps that detect voltage differences between the bit lines and a capacitor, a process that makes DRAMs relatively slow. The new design uses a three-transistor micro-sense amp that lets voltage current directly drive transistor gates.

IBM used embedded DRAM in a custom processor designed for its high-end Blue Gene/L supercomputers, but has not been able to use the technology in mass market computer chips to date. "This is 100 percent mainstream and we expect to get it in products in 2008," Iyer said.

Intel and other chip makers are investigating using the floating body cells to store charge as one alternative for embedded memory. Other chip makers are researching stacking memory and processor dice in multi-chip modules.

Intel archrival Advanced Micro Devices co-develops process technology with IBM and could use the embedded DRAM technology as a way to compete with Intel.

- Rick Merritt
EE Times

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